When booking a ticket, regardless of whether it is using a published or unpublished fare, there are letters that are assigned to different fares. These letters indicate the class of service, not simply indicating economy or business class, but rather the type of fare that was purchased. Without getting into the particular rules of each fare type, letters are assigned by the airlines to indicate the kind of ticket that has been purchased.
F, P are the letters most commonly used to indicate First Class.

J, C are the letters most often used to represent Business, or Executive Class.

Y is almost universally used to indicate a full fare economy ticket.

B, H, L, M, V, etc. are just some of the letters indicating subclasses (reduced, restricted, and/or discounted fares). These letters vary by airline and in value. On one airline B may be indicative of a more expensive ticket. On another airline L may represent a ticket booked on a seat sale.

X, U, R are a few of the letters commonly used to indicate a fare purchased from a consolidator.

If you have booked a discounted fare and are interested in the subclass you are holding, the letter immediately follows the flight number on your ticket and will fall under the heading of Booking Class or some similar, shortened version.